The invention relates to an improved process for the preparation of crystalline iron silicates.
Mixtures of carbon monoxide and hydrogen can be converted into aromatic hydrocarbon mixtures using a mixture of two catalysts of which one has the capability of catalyzing the conversion of an H.sub.2 /CO mixture into acyclic oxygen-containing hydrocarbons and the other a crystalline iron or aluminum silicate having the capability of catalyzing the conversion of acyclic oxygen-containing hydrocarbons into aromatic hydrocarbons.
As described in U.S. Pat. No. 4,208,305 incorporated by reference, the crystalline silicates which are used in the catalyst mixtures may be prepared starting from an aqueous mixture containing the following compounds: one or more compounds of an alkali metal (M), one or more quaternary alkylammonium compounds (R.sub.4 NX), one or more silicon compounds and one or more iron or aluminum compounds.
The silicon compounds which are used in the preparation of the crystalline silicates may, on the basis of the SiO.sub.2 content of the product that is obtained from these compounds after drying at 120.degree. C. and calcining at 500.degree. C., be divided into two classes, viz. silicon compounds yielding a product with an SiO.sub.2 content of more than 90%w and silicon compounds yielding a product with an SiO.sub.2 content of at most 90%w. For the sake of brevity, the silicon compounds which are suitable for use in the preparation of the crystalline silicates will, on the basis of the above-mentioned division, further be designated in this patent application as silicon compounds with a high and with a low SiO.sub.2 content, respectively. Examples of silicon compounds with a high SiO.sub.2 content are amophous solid silicas, silica sols, silica gels and silicic acid. An example of a silicon compound with a low SiO.sub.2 content is water glass. The preparation of the crystalline silicates is performed by maintaining the mixture at elevated temperature until the crystalline silicate has been formed, separating it from the mother liquor and calcining it. In the aqueous mixture from which the silicates are prepared the various compounds should be present in the following ratio, expressed in moles of the oxides:
M.sub.2 O: SiO.sub.2 =0.01-0.35, PA1 (R.sub.4 N).sub.2 O: SiO.sub.2 =0.01-0.4, PA1 SiO.sub.2 : A.sub.2 O.sub.3 &gt;10, and PA1 H.sub.2 O: SiO.sub.2 =5-65. PA1 M.sub.2 O: SiO.sub.2 =0.01-0.35, PA1 R.sub.1 R.sub.2 R.sub.3 N: SiO.sub.2 =0.04-1.0, PA1 SiO.sub.2 : Fe.sub.2 O.sub.3 &gt;10, and PA1 H.sub.2 O: SiO.sub.2 =5-65, PA1 (a) thermally stable up to a temperature above 600.degree. C., PA1 (b) an X-ray powder diffraction pattern showing, inter alia, the reflections given in Table A of the specification, and PA1 (c) in the formula that represents the composition of the silicate, expressed in moles of the oxides, and in which, in addition to oxides of hydrogen, alkali metal and silicon, there is present an oxide of a trivalent metal A selected from Al and Fe, the SiO.sub.2 /A.sub.2 O.sub.3 molar ratio (for the sake of brevity further designated as m in this patent application) is more than 10.
Although according to the above-mentioned procedure crystalline silicates with excellent catalytic properties can be prepared, the procedure has nevertheless a serious drawback, which is connected with the use of the quaternary alkylammonium compound and which drawback hampers use on a technical scale. Quaternary alkylammonium compounds are relatively costly compounds, which are virtually unsuitable for use in a catalyst preparation on a technical scale, in view of the quantities required in this procedure.
Applicant's have carried out an investigation concerning the preparation of the above-mentioned crystalline iron silicates, in which instead of quaternary alkylammonium compounds, amines with the general formula R.sub.1 R.sub.2 R.sub.3 N are used, in which R.sub.1 represents an alkyl group and R.sub.2 and R.sub.3 represent an alkyl group or an hydrogen atom as shown, e.g., in silicate 22 of the aforesaid U.S. Pat. No. 4,208,305, employing a compound of low silicon content. In an investigation concerning the use of the above-mentioned catalyst mixtures, in which as the crystalline silicate there has been incorporated an iron silicate prepared using an alkylamine instead of a quaternary alkylammonium compound, for the preparation of aromatic hydrocarbon mixtures starting from H.sub.2 /CO mixtures with an H.sub.2 /CO molar ratio below 1.0, it has been found that the stability of these catalyst mixtures is determined to a large extent by the nature of the silicon compound present in the aqueous mixture from which the crystalline iron silicate component of the catalyst mixture is prepared. It has been found that catalyst mixtures with excellent stability for the conversion of H.sub.2 /CO mixtures with an H.sub.2 /CO molar ratio below 1.0 into aromatic hydrocarbon mixtures can be obtained by using in the catalyst mixtures a crystalline iron silicate component prepared from an aqueous mixture in which one or more amines with the general formula R.sub.1 R.sub.2 R.sub.3 N and one or more silicon compounds with a high SiO.sub.2 content are present. The preparation of crystalline iron silicates having the properties mentioned hereinafter under (a)-(c), starting from an aqueous mixture in which one or more amines with the general formula R.sub.1 R.sub.2 R.sub.3 N and one or more silicon compounds with a high SiO.sub.2 content are present, is novel.